So as the title states, the engine would start and run for about a minute and then die. And I could start it back up again n it would run and then die.
now the backstory:
Bought the jeep back in 2020. Had a cracked head. Pulled the engine and had it rebuilt. I sourced a good head off a 97 or 98 Cherokee. Had the engine rebuilt and put a cam n timing kit from Cleeg put in it and I bought a hopped up coil, cap, rotor and plugs from Morris 4x4. At the end of 2021 I moved to SoCal from Oregon. I had my buddy bring my Jeep and engine down after the engine work was done. Someone broke into my Jeep and stole all my aluminum engine parts, cam, pistons/rods and anything else that was in my tote. So I lost the intake, timing cover, engine mounts, AC mounts and distributor. So I get a engine for a 97 Grand Cherokee for engine parts and everything worked but the engine mounts and ac mount(naturally lol). Other then the theft, everything went together perfect during the whole build.
so I get the engine in, turn it over and it fires on the first spin. Run up the RPM to break in the cam and it runs for like 2 mins and dies. Cycle the key and it starts right up again. I test fuel pressure and I'm sitting above 40psi, so I go to the CPS. Buy a cheap one from O'Reilly's (I know, I know, first mistake. I know aftermarket parts are junk. I'm a full time mechanic lol) thing runs for a lil bit, then dies. But now the thing won't start at all. Just turns over. I purchase a Jeep CPS from my local dealer and put that in. Nothing, put the old factory(kind of damaged) CPS in, nothing. I'm not even getting any movement of my tach needle at all. I put a spark tester in-between the coil wire and a plug wire. No spark from either. Put the original coil back in, nothing. I'm at a lost. Even my bestie who worked for Dodge/Jeep for 25 years is lost. I'm about to test the wave form from my CPS and then test it coming out my ECU next. I would hate to thing the ECU just took a crap one. But I can't think of anything else.
thanks for making it this far lol
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My first thought is if you have the original ECU for the 1996 Jeep it may of shut down the ASD circuit because it was reading information it doesn't understand. You may want to source a ECU to match the motor that you installed and see if that makes a difference.
This is a guess from a shade tree wrencher. You may want to ask people who know a great more about it than I do.
Hope this helps

The block, crank and sensors all came from the Jeep originally. The only things that came from the donor engine were the timing cover and intake.

Do you have the complete 1996 FSM and the Technicians Drivetrain Diagnostic Manual ?

Have you conducted an OBD2 scan (BAFX works on '96, cost $30, mine is always plugged in

I am betting you do not have either, but you need them to help diagnose your issue

For a shortcut, put your fingers on the ASD and fuel pump relay, to see if they click in when they should

I am a qualified electronics technician and downloaded both those resources for free of the Internet

I am not able to comprehend how anyone could go about complex fault finding one without those manuals(

(..it saved my **** when it stated the front brake caliper bolt should be no longer than 66.7mm, and some PO must have lost one, and put in a 70mm replacement, which is stock replacement bolt size, new pads caused the caliper to lock on sometimes, I had tried the various problems that cause this. Without the FSM, I never would have found the cause, shame I did not consult it earlier)

Own 3 x'96, one is my DD, and I keep them all going, I will try and help

I have a FSM for a 97 Cherokee. My jeep is a JTEC style ECU, so the only thing I've noticed being different is some wire colors. I also have an scan tool(Snap on Modus and Launch) and access to 3 other types of scan tools from my neighboring shops and an oscilloscope for reading waveform.
LIke I said in my original post, I'm a full time mechanic with over 25+ years experience. This isn't my first Jeep and I've read almost every post in here about no start issues.
To answer your question, yes I can feel both relays kick on when I turn on the key. I also have fuel pressure when I turn on the key.

My apologies. I start with the basics, but do not wish to seem condescending

(You did not actually mention you were a mechanic, or that you had worked Jeeps)

There are a few problems that have arisen on this forum that may cause your issue

Let us concentrate on no spark

There is a detailed "no spark" diagnostic in the FSM, and a flowchart in the Drivetrain Diagnostic Manual

As you know, the PCM controls ignition by providing a path to ground, so you have the issue of trying to find out whether the problem is a lack of 12V to coil via bad ignition switch or wiring, or loss of earth path due to bad wiring or failing Mosfet in PCM

The wiring harness in these older Jeeps (and much newer ones) are quite suspect, and moving them around enough to replace engine can cause an intermittent failure, the worst kind to diagnose

When it quits, does it lose electrical power elsewhere ?

You mention no tacho, that may or may not be relevant

I assume you have no OBD2 codes present

Fuel pressure on a '96 should be 49psi

Did you change the flexplate ?

I am pretty sure there is a test outlined in the FSM to test whether you get spark by providing an alternative ground path. If you can, its either the wires to the PCM, or the PCM spark circuitry

I will try and have a look for that later

A '96 will only work properly with a '96 PCM, they can be bought in USA, I linked it up for another 96sixer recently $350 I think

I tried looking for a diagnostic drivetrain manual but only found one from someone on here for a 91-95 jeep. So it's old renix and not the same ecu. I'll look in my FSM tomorrow for the no start trouble tree when I go tinker on the Jeep.
1. I do not lose power everywhere. The engine just shuts off. Well it did. The engine won't even try to start now.
2. When I checked fuel pressure before it was close to 50. I'll check again and see what I get now.
3. No codes currently. My buddy had me disconnect the CPS to see if it would throw a code and it did for not seeing a crank signal.
4. I have not replaced the flex plate. I used my old one from the original engine. I thought I may have accidentally put the flex plate from my donor ZJ engine on there, but according to the pictures I found on Rock Auto, I did in fact put my XJ one in.
5. Don't think there was a 5th question, but I'm not hitting the back button to look again lol

That will be me, download it !

The reason being it is written for the actual mechanics at the time by Chrysler to help them and the flowchart for lack of ignition will be the same

(with one possible difference, not 100% sure if OBD1 provides earth or + at the PCM to fire ignition..pretty sure 94-95 OBD1 is the same as '96 OBD2 in this respect)

I can probably find and post up the diagnostic in the '96 PCM for no spark, but it will be the same a s"97, which I think you mention you have

ask away if u want me to post it up

If it helps, here are 2 links.

The first has service and parts manuals for most years of XJs. You'll find the '96 FSM here but not the parts manual.

https://archive.org/details/addition...?query=xj+jeep

This one has some of the same service manuals but has the '96 parts manual ('94-'96).

https://xjjeeps.com/

There are other useful documents on each site.

Thanks for the manual then :⁠-⁠) reading through my 97 FSM, it looks like the coil is still driven by the PCM by use of negative control. So I can use either book. I did want to ask if you had a copy of the 97 Powertrain Diagnostic Procedures manual by chance?

Relevant excerpt from '96 FSM






Excerpts from 95 Diagnostic, I have only ever seen the 97 on ebya as a hrad copy, but as you can see from the flowchart, it will be the same except different pin nos for obd1







As it shows its almost brutally simple, no 12V at coil and it something other than a bad wire or PCM

If you get spark upon providing an alternative earth path, it is a wire, connector or PCM

I had a slightly disconnected PCM pin in my spare '96, no voltage regulation, **** knows how that happened

I don't really need the capacitor in the jumper wire do I lol

Reasonable question

It is not marked for polarity, so you probably will not do any harm

The other functions of a capacitor are to mute higher voltage ripples, or store a charge

So its slightly possible the hoped for "spark induction" would be weak or not present

So you would get a false negative reading on a failed PCM

If you do not have one readily available, I would perform the test without

If the test is a fail, I would obtain said capacitor and try again prior to buying a new PCM, or setting fire to my XJ

If you read back in the archives, you will see that the PCM can be removed from its metal case to check for bad internal pins, or a burnt area that will indicate a fail

Some folk have even repaired them, the failure can be and often is, a capacitor on the PCM board, its encased in gel, so a reman is easier

One thing you can try is make a piggyback wire with backprobes that runs together with the connected wire. I fixed an issue this way once, had me stumped, I got lucky after 50yrs of experience

Well doing the jumper test created a spark. I went through the whole no spark test from the FSM and everything checked out. Did the jumper thing last and it created a spark!! I found a couple people on eBay who rebuild these ECU's. I'm probably going to open mine up before sending and see if I can find a problem that I can fix myself. I have a soldering iron just for circuit boards because I have repaired more gm abs units then I can remember lol

A little update for everyone so the thread doesn't just end with no conclusion LOL ended up sending the ECU to a company back in the Midwest and they ended up replacing a couple capacitors and fixing the coil driver circuit within the ECU. I found them on eBay and the prices for a reasonable($115, I paid like $18 to so it from SoCal) Fingers crossed that actually fixes my problem and the Jeep starts whenever I get the ECU back next week.